。
She studied English in the morning.
Literal
She [topic-は] morning-during [in-に] English [object-を] studied.
午前中 ('during the morning') uses the suffix ~中 (ちゅう) to mark a span of time — 'over the course of morning.' The に here pinpoints when the study took place, anchoring the morning as the relevant timeframe. 英語を勉強する is the standard collocation for 'study English' — Japanese frames learning a language as 'studying' rather than 'taking a class' or 'practicing.' English instruction starts in elementary school under current curriculum and continues through high school.